1. Field of the Invention
The present invention relates to a diaphragm type pressure sensor and, more particularly, to the diaphragm type pressure sensor which is hardly influenced by the change of ambient temperature and makes possible precise pressure measurements with a high accuracy and sensitivity.
2. Description of Related Art
A conventional capacitive pressure sensor is explained below as an example of diaphragm type pressure sensor.
As shown in a schematic diagram of FIG. 6, a capacitive pressure sensor has a configuration in which a first glass substrate 10 on which a capacitance electrode 11 is formed, a silicon substrate 20 in which a diaphragm 21 is formed, and a second glass substrate 30 through which a gas inlet 31 is formed, are bonded to form a pressure reference room 1 and a pressure measuring room 2 on both sides of the diaphragm 21. A non-evaporable getter 3 is placed inside the pressure reference room 1 to maintain the room at a high vacuum.
The pressure sensor is mounted on, for example, an adapter 4 with an O-ring 5, and is fixed by using a press plate 6. The adapter 4 is installed in a gauge port of a vacuum chamber. The vacuum chamber is communicated with the pressure measuring room 2 through the gas inlet 31. The pressure in the vacuum chamber is applied on the pressure measuring room to deform the diaphragm. The amount of deformation depends on the pressure difference between the pressure measuring room 2 and the pressure reference room 1, and therefore can be obtained by measuring electrostatic capacitance between the diaphragm 21 and the capacitance electrode 11 with the aid of terminal pins 12 and 13.
A micromachining technique is used for manufacturing such pressure sensors.
An anodic bonding method is usually used to bond the first and second glass substrates to the silicon substrate. Here, the silicon substrate and the glass substrate are heated to 300-450° C. and a voltage of several hundreds volts to 1000 volts is applied between the substrates to firmly bond the substrates.
However, when the substrates are cooled to a room temperature after bonded at such a high temperature, the distortion appears in the diaphragm due to the difference in thermal expansion rate characteristics between the silicon substrate and the glass substrate. As a result, the stability of the pressure measurements is decreased and measured values fluctuate largely when the ambient temperature around the sensor is changed. For these reasons, glasses such as a Pyrex glass (Corning Inc.), having a thermal expansion rate characteristic close to that of silicon, have been generally employed to make the diaphragm distortion minimum. The conventional sensors are described in, for example, JP2001-255225A and JP1994-66658A.